1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. Specifically, the present invention relates to a technique for reducing a punch-through current in a thin film transistor (TFT) over a silicon on insulator (SOI) substrate formed using a bonding technique.
2. Description of the Related Art
In recent years, miniaturization of a semiconductor element such as a TFT has been developed with improvement in characteristics of an integrated circuit. However, various problems are caused by microfabrication in a submicron region.
As one problem, a phenomenon called a short-channel effect is typically known. The short-channel effect is a phenomenon, in a semiconductor element having a channel formation region between a source region and a drain region, caused because a charge in a channel formation region is largely affected not only by a gate voltage but also by a depletion layer charge, an electric field, and electrical potential distribution in a source region and a drain region, as the channel formation region is shortened.
In addition, as an effect on a semiconductor element by the short-channel effect, reduction in a threshold voltage (Vth) and a punch-through current are known.
By reducing the threshold voltage, power consumption can be reduced. However, since frequency characteristics do not generally become high by reducing a driving voltage of an integrated circuit, as a method for controlling the threshold voltage, a method for adding an impurity element imparting one conductivity type into the entire channel formation region to control the threshold voltage by the added amount of the impurity element, is employed. However, along with this, there are problems that a punch-through current is generated and carriers are scattered by the added impurity to reduce mobility of the carriers.
Furthermore, it is known that when the effect of a gate voltage to a drain current is reduced due to the punch-through current, subthreshold characteristics become worse and switching characteristics of a semiconductor element is deteriorated. As a method for suppressing the punch-through current, there is a method in which a channel formation region is formed to be thin; however, such a problem occurs that when the channel formation region is formed to be thin, resistance of a source region and a drain region is increased and an ON current of the semiconductor element is decreased.
As another method for suppressing the punch-through current, there is a method in which an impurity element imparting a conductivity type opposite to that of a source region and a drain region is implanted into a bottom portion of the channel formation region; however, it is difficult to implant the impurity element only into the bottom portion of the channel formation region from the surface of the channel formation region.
On the other hand, a method is known in which after an impurity element imparting a conductivity type opposite to that of a source region and a drain region is implanted into a silicon substrate using an SOI technique, the silicon substrate is bonded to a base substrate and polished, thereby forming an impurity implanted region for suppressing a punch-through current in a bottom portion of a channel formation region of a semiconductor element formed over the base substrate (For example, see patent document 1: Published Patent Application No. H5-326962 and patent document 2: Published Patent Application No. H7-142738).
However, in the case of applying such a method, since the impurity element is implanted into the silicon substrate, after the silicon substrate is bonded to the base substrate, the silicon substrate which is separated from the base substrate cannot be reused efficiently, and there is a problem of saving resources.